Flat panel displays typically convert image data into varying voltages fed to an array of picture elements (pixels) causing the pixels to either pass light from a backlight as in a liquid crystal display (LCD), or to emit light as in for example an electroluminescent, LCD display, or organic light emitting diode (OLED) display. The image voltages determine the amount of light from the pixel. Active matrix displays generally include an array of pixels arranged in a row-and-column format, each pixel contains a sample and hold circuit plus, in the case of pixel light emission displays, a power thin film transistor (TFT). One advantage of the active matrix is that each line of the display is held on for the full frame length so that the instantaneous brightness of the pixels is close to the average brightness. This is not true of passive displays since they are on only one line at a time; therefore, each line must have an instantaneous brightness equal to the average brightness multiplied by the number of lines. The active matrix display generally has a longer life time, lower power consumption and is capable of many times the line capability of the passive display. In general all full color monitor, laptop and video flat panel displays employ the active matrix while low resolution monochromatic (area color—icons) are passive.
In the case of the active matrix OLED display, a voltage is placed on the gate of a power transistor in the pixel, which feeds current to the OLED pixel. The higher the gate voltage, the higher the current and the greater the light emission from the pixel. It is difficult to produce uniform pixels and even if such uniform pixels could be produced it is difficult to maintain uniformity during the lifetime of a display containing an array of such pixels. As a result of manufacturing tolerances, transistor current parameters typically vary from pixel to pixel. Also the amount of light emitted by the OLED material varies depending on the OLED's current-to-light conversion efficiency, the age of the OLED material, the environment to which individual pixels of the OLED-based display are exposed, and other factors. For example, the pixels at an edge of the OLED display may age differently than those in the interior near the center, and pixels that are subject to direct sunlight may age differently than those which are shaded or partially shaded. In an attempt to overcome the uniformity problem in emissive displays, several circuit schemes and methodologies are in use today. One scheme uses a current mirror at the pixel where, instead of image voltages, image currents are used to force a particular current through the power transistor feeding the OLED. Also circuits have been designed which test the power transistor threshold voltage and then add the image voltage to the threshold voltage, therefore, subtracting out the threshold voltage so that variances in threshold voltage do not vary the OLED brightness. These circuit schemes are complex, expensive to produce and have not been entirely satisfactory.
Any display that requires a large number of gray shades requires uniformity greater than one shade of gray. For example, a hundred shades of gray require a display uniformity of 1% in order to use one hundred brightness levels. For a thousand gray levels 0.1% brightness uniformity is desired. Since it is difficult, if not impossible, to have a mass production process that holds 0.1% uniformity in the thin film area, another means of forcing uniformity on the display must be found.
One previous approach was to use certain optical feed back circuits, providing a particular type of feedback from optical diodes or optical transistors in an attempt to provide data on the actual brightness of a pixel's light emission and use the fed back data to cause a storage capacitor to discharge, thus, shutting down the power transistor. This requires a photodiode placed at each pixel as well as a means of reacting to the data supplied by the photodiode. Each pixel must have the discharge circuit. Accordingly, each pixel must include a highly complex circuit. Further, the circuit elements themselves, including the photodiode all introduce variables, which introduce non-uniformity. Further this approach only tends to cause uniformity since bright pixels are shut down faster and dim pixels are left on longer, but no exact brightness level is measured or used as a reference.
A second approach added a blocking transistor to the optical diode that relied on the pixel reaching an equilibrium brightness determined by the pixel brightness, the optical response of the diode, and all the parameters that determine the current supplied by the power transistor during the write time of the image line. However, the equilibrium brightness is determined by all the parameters mentioned above and these parameters can vary from pixel to pixel. Therefore, the attempted correction was not pixel-specific and did not take into account the changes for each pixel over time. Another problem is that the particular feedback circuit and method can set the system into oscillations, which if not damped within the line write time, would leave the actual brightness and voltage undetermined at the point of write time cut off.
Accordingly, an apparatus, system. and method is needed that stabilizes a display but advantageously is not effected by variation in photodiodes or other circuit parameters. The apparatus, system, and method should preferably not allow the system to enter oscillation and should allow the full range of brightness to be used over the life of the display.